Update article Sensorimotor transformations in cortical motor areas Shinji Kakei a,b, *, Donna S. Hoffman d,f , Peter L. Strick c,d,e,f a Systems Neuroscience, Graduate School of Life Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b PRESTO, Japan Science and Technology Corporation (JST), Japan c Research Service, Veterans Affairs Medical Center, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA d Department of Neurobiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA e Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA f Center for the Neural Basis of Cognition, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA Received 10 October 2002; accepted 20 January 2003 Abstract A central problem in motor research has been to understand how sensory signals are transformed to generate a goal-directed movement. This problem has been formulated as a set of coordinate transformations that begins with an extrinsic coordinate frame representing the spatial location of a target and ends with an intrinsic coordinate frame describing muscle activation patterns. Insight into this process of sensorimotor transformation can be gained by examining the coordinate frames of neuronal activity in interconnected regions of the brain. We recorded the activity of neurons in primary motor cortex (M1) and ventral premotor cortex (PMv) in a monkey trained to perform a task which dissociates three major coordinate frames of wrist movement: muscle, wrist joint, and an extrinsic coordinate frame. We found three major types of neurons in M1 and PMv. The first type was termed ‘extrinsic-like’. The activity of these neurons appeared to encode the direction of movement in space independent of the patterns of wrist muscle activity or joint movement that produced the movements. The second type was termed ‘extrinsic-like with gain modulation’. The activity of these neurons appeared to encode the direction of movement in space, but the magnitude (gain) of neuronal activity depended on the posture of the forearm. The third type was termed ‘muscle-like’ since their activity co-varied with muscle activity. The great majority of the directionally-tuned neurons in the PMv were classified as ‘extrinsic-like’ (48/59, 81%). A smaller group was classified as ‘extrinsic-like with gain modulation’ (7/59, 12%). In M1, the three types of neurons were more equally represented. Our results raise the possibility that cortical processing between M1 and PMv may contribute to a sensorimotor transformation between extrinsic and intrinsic coordinate frames. Recent modeling studies have demonstrated the computational plausibility of such a process. # 2003 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved. Keywords: Motor cortex; Ventral premotor cortex; Coordinate transformations 1. Introduction To generate a goal-directed movement, the brain must translate the location of a target into a set of muscle activation patterns. A central problem in motor research has been to understand how the process of sensorimotor transformation is accomplished by the central nervous system. The solution is thought to involve a set of transformations between the coordinate frames or reference frames for movement representation. A co- ordinate frame describes the measurement system which encodes specific movement variables (Soechting and Flanders, 1992). Two general types of coordinate frames can be described: extrinsic and intrinsic. An extrinsic coordinate frame is fixed to external space and is independent of body movement. In contrast, an intrinsic coordinate frame is related to and moves with a specific body part such as a joint or muscle. Each type of coordinate frame provides a specialized description of movements and has unique advantages. For instance, an extrinsic coordinate frame is essential to specify the location of a target in space, whereas a joint coordinate frame provides the most concise description of the position of a limb. It is likely that the nervous system * Corresponding author. Tel.: /81-22-217-5047; fax: /81-22-217- 5048. E-mail address: s-kakei@mail.cc.tohoku.ac.jp (S. Kakei). Neuroscience Research 46 (2003) 1 /10 www.elsevier.com/locate/neures 0168-0102/03/$ - see front matter # 2003 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved. doi:10.1016/S0168-0102(03)00031-2